Texture appearing in squares across the map
I would start by looking at your UV map. If you try using the color grid or UV preset textures (when you create a new texture) you should be able to see if your UV map makes sense, or if it's causing the issue.

If the UV map is the problem, you can probably just UV unwrap your object again. To make a flat UV map from above your object (which should work for your terrain), switch to edit mode with Tab, toggle to orthographic view with Ctrl+5, switch to top view with Ctrl+7,  then select UV unwrap (or press U) > Project from View (Bounds).